Are you claiming that gender and sex were only used interchangeably in 2010??
The idea that there is a bunch of socially-constructed stereotypes that get attached to biological males, and biological females, and that vary from culture to culture, class to class and historical era to historical era, goes way back beyond 2010.
Back in the last century, feminists used the concept of gender as opposed to biological sex to critique those gender stereotypes, not to celebrate them as a multiplicity of genders with their own flags!
The whole point about the feminist analysis of sex and gender is that sex is innate and immutable, but gender is not, and gender stereotypes can and should be changed; whereas 'genderwoo' claims that gender is innate and immutable, but sex can be changed.
The words 'sex' and 'gender' have been used interchangeably in everyday speech on both sides of the Atlantic for a long time. But there has also been a use of the terms separately in discussions biological sex vs. gender stereotypes for a long time.
Talulahalula has already posted a more succinct version of what I was trying to say - I have difficulty with words sometimes, and it often takes me a long time to write a post [which always turn out to be too long 😐], but I've spent so fecking long writing it that feck it, I'm posting this even though you've already said it so much better😄